Electric Flight Continues to Spread Its Wings

By Bill Moore

Posted: 08 Nov 2009

Two events took place recently in the march towards practical electric flight. The first was the four-motored Solar Impulse was rolled out of its hangar in Switzerland for electric motor run-up tests.

The other was somewhat less dramatic, but no less important. A LAK17A, 18m-class sailplane successfully flew with an Slovenian-developed electric propeller system (see the YouTube video below). Developed by the father-son team of Luka and Matija Znidarsic in Ljubljana, Slovenia, the Front Electric Sustainer (FES) incorporates a folding 0.9m carbon fiber propeller that automatically unfolds using centrifugal force. When not in use, the props fold against the nose of the sailplane, reducing their drag. Each blade weighs just 110 grams.

The Znidarsic's developed their own brushless DC synchronous permanent magnet electric motor that develops 15kW of continuous power at 85V. It weighs just 5 kg (11 lbs. ) and is rated at 95% efficient. Their controller, also self-developed, is mounted atop the sailplane's main wheel well box just behind the single-seat cockpit.

They turned to Kokam for their lithium-ion batteries. Mounted in to battery boxes behind the cockpit and accessible through panel in the stop of the fuselage, each 10 cell pack weighs 11 kg. The total available energy is 3.6kWh. In addition to powering the FES, they also provide electric energy for the plane's instrumentation through a DC-DC converter. Voltage range of the battery system is 60-85 volt. The batteries can be removed for recharging, which takes 4 hours.

The Znidarsics explain they that they chose to develop the front -mounted system rather than the retractible pylon-type used by Pipistrel -- also located in Slovenia -- based on experience with model aircraft. They explain on their web site:

In my youth, I mounted a small Cox combustion engine above the fuselage of a light 2m wingspan RC sailplane. The first flight tests were very disappointing as the model barely flew horizontally under full power. After I installing the same engine in the front part of fuselage, the difference was unbelievable as the model climbed very well.
They also note that the front sustainer approach is less expensive, lighter and provides better handling than the competing retractable pylon system.

While the FES-equipped LAK17A still requires a tow to launch, it can release sooner, reducing the cost of the tow, and then climb to altitude at a rate of 1.5m per second. Low-speed cruising time is about a hour, they estimate. As the video demonstrates, the motor can be instantly turned on to allow the pilot to extend his glide path if he's misjudged his landing.

The family is working on European certification of their system and see it eventually being adapted to a wide range of high-performance sailplanes including LAK19 and the new LAK17B, as well as other models from other manufacturers. Pricing was not announced on the web site as of this writing.

LAK17A fitted with FES electric propeller

Full view of LAK17A with FES-developed electric flight sustaining system.

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